We plan to characterize further the nature of the sodium and potassium transport systems in the toad urinary bladder. First, by methods previosuly described, we will investigate the kinetics of sodium transport in this system, with particular attention to the effects gf anions, temperature changes, and pH changes on the sodium pump and the sodium transport pool. We shall also focus on the stoichiometry of Na-K exchange by the pump, since our method offers an opportunity to make simultaneous determinations of the unidirectional fluxes of both ions at the serosal border of the cells, where the active transport mechanism is located. Secondly, we will examine, with the same technique, the factors which control sodium entry into the cells at the mucosal border. For instance, we have shown that ouabain inhibits sodium entry under certain conditions. We plan to characterize this inhibition further, with particular emphasis on the effect of pH, anionic content of the media, and changes in the transmembrane electrochemical potential gradient for sodium on the entry step. Thirdly, we have recently determined the passive electrical properties of this system, including quantitation of the resistances of the cellular and paracellular transepithelial pathways, as well as a determination of the nature of cell-to-cell electrical coupling. We shall investigate these pathways further, with particular emphasis on those factors which appear to control the relative contributions of the two transepithelial pathways. These experiments will also involve visual determinations of cell coupling by the use of iontophoretic injection of dyes under different conditions.